Reducing codec noise in acoustic devices

ABSTRACT

The technology described in this document can be embodied in a computer-implemented method that includes receiving, at a first acoustic device, a representation of an audio signal, and amplifying the representation of the audio signal by a first gain factor to generate an amplified input signal. The method also includes processing the amplified input signal by an audio codec that includes one or more processors to generate a processed signal that represents a portion of the audio signal to be output by a second acoustic device. The processed signal includes noise originating at the audio codec. The method further includes transmitting the processed signal to the second acoustic device.

TECHNICAL FIELD

This disclosure generally relates to acoustic devices that employ audiocodecs for encoding and decoding audio signals.

BACKGROUND

Acoustic devices such as wireless earphones or headphones can includeaudio codecs for encoding and decoding audio signals. In some cases, anaudio codec may act as a noise source by introducing noise in the signalprocessed by the codec.

SUMMARY

In one aspect, this document features a computer-implemented method thatincludes receiving, at a first acoustic device, a representation of anaudio signal, and amplifying the representation of the audio signal by afirst gain factor to generate an amplified input signal. The method alsoincludes processing the amplified input signal by an audio codec thatincludes one or more processors to generate a processed signal thatrepresents a portion of the audio signal to be output by a secondacoustic device. The processed signal includes noise originating at theaudio codec. The method further includes transmitting the processedsignal to the second acoustic device.

In another aspect, this document features a system that includes a firstacoustic device configured to receive a representation of an audiosignal. The first acoustic device includes an amplifier configured toamplify the representation of the audio signal by a first gain factor togenerate an amplified input signal. The first acoustic device alsoincludes an audio codec having one or more processors, the audio codecconfigured to process the amplified input signal to generate a processedsignal that represents a portion of the audio signal to be output by asecond acoustic device. The processed signal includes noise originatingat the audio codec. The first acoustic device also includes atransmitter for transmitting the processed signal to the second acousticdevice.

In another aspect, this document features a machine-readable storagedevice having encoded thereon computer readable instructions for causingone or more processors to perform various operations. The operationsinclude receiving a representation of an audio signal, and amplifyingthe representation of the audio signal by a first gain factor togenerate an amplified input signal. The operations also includeprocessing the amplified input signal to generate a processed signalthat represents a portion of the audio signal to be output by anacoustic device, and providing the processed signal for transmission tothe acoustic device. The processed signal includes noise originating atan audio codec.

Implementations of the above aspects may include one or more of thefollowing features.

A representation of the processed signal may be received at the secondacoustic device, where the first gain factor can be compensated for byamplifying the representation of the processed signal by a second gainfactor to generate a compensated signal. The compensated signal can beoutputted at the second acoustic device. The second gain factor can besubstantially an inverse of the first gain factor. The audio codec canbe an adaptive differential pulse code modulation (ADPCM) codec. Theprocessed signal can also include noise originating at an asynchronoussample rate converter used in conjunction with the audio codec. Thefirst gain factor can be selected such that the amplified input signaldoes not exceed a dynamic range of a system in which the audio codec isdisposed. The amplified input signal can be compressed in accordancewith a dynamic range of a system in which the audio codec is disposed.The first gain factor can be selected adaptively in accordance with thereceived representation of the audio signal. A dynamic range of thereceived representation of the audio signal can be detected, and anavailable dynamic range headroom can be determined as a differencebetween a dynamic range of the system, and the dynamic range of thereceived representation of the audio signal. The first gain factor canbe selected such that an associated amount of amplification does notexceed the available dynamic range headroom. The information indicativeof the adaptively selected gain factor can be transmitted to the secondacoustic device. The first and second acoustic devices can be earphonesthat are connected to one another over a wireless link. The first gainfactor is a function of frequency.

Various implementations described herein may provide one or more of thefollowing advantages.

By pre-processing an input signal to a codec by applying a gain factor,and compensating for the additional gain at the output of the codec, theeffect of codec-introduced noise may be mitigated, thereby reducing theneed for computationally intensive noise reduction techniques. This inturn may improve the performance of codecs used for satisfying bandwidthrequirements in transmitting audio signals from one acoustic earbud toanother over a wireless channel. In addition, by adjusting the gainfactor in accordance with available dynamic range headroom, any signaldegradation due to a limitation in the number of bits of the codec maybe mitigated. This in turn may be used to support higher dynamic rangecontent (e.g., high dynamic range music) without having unacceptablelevels of degradation in audio quality.

Two or more of the features described in this disclosure, includingthose described in this summary section, may be combined to formimplementations not specifically described herein.

The details of one or more implementations are set forth in theaccompanying drawings and the description below. Other features,objects, and advantages will be apparent from the description anddrawings, and from the claims.

DESCRIPTION OF THE DRAWINGS

FIG. 1 depicts a schematic diagram illustrating signal paths in anexample system that includes a set of wireless acoustic earbudsreceiving audio from a source device.

FIG. 2 is a block diagram of the system depicted in FIG. 1.

FIGS. 3A and 3B show plots illustrating degradation of signal due tocodec noise.

FIGS. 4A-4C are block diagrams of systems for reducing codec noise.

FIGS. 5A and 5B show plots illustrating reduction of codec noise.

FIG. 6A illustrates dynamic range reduction due to codec usage.

FIG. 6B illustrates improvement in dynamic range reduction.

FIG. 7 is a flowchart of an example process for reducing codec noise.

DETAILED DESCRIPTION

Wireless acoustic earbuds that are connected to one another, as well asto a source device (e.g., a phone, tablet, or other media player), overrespective wireless channels have become popular as personal audiodevices for various reasons such as aesthetic appeal and ease of useduring various activities. The popularity of such acoustic systems is inpart attributable to the lack of wires connecting the various componentsof the system. The lack of wired connections however creates otherchallenges, such as maintaining quality of the audio signals transmittedbetween the components. For example, transmitting audio signals from oneacoustic earbud to another through a low energy wireless channelestablished through the head of the user can be particularlychallenging. In some implementations, this may entail additionalprocessing by audio codecs such as ITU G.722. Such codecs can compressthe bit rate of audio signals, but sometimes at the cost of introducingcodec-generated noise and/or adversely affecting the dynamic range ofthe audio signals. This document describes various technologies forimproving the quality of audio signals, for example, by reducing adverseeffects of codec generated noise, and/or by reducing adverse effects ofbit rate compression on dynamic ranges of the signals.

The two different types of compression described should not confusedwith one another. The first type, which may also be referred to as audiocompression, is a data compression technique used for generatingcompressed audio. Audio compression involves reducing the amount of datacorresponding to audio waveforms to different degrees (e.g., dependingon types of audio compression processes, and/or whether the audiocompression is lossy or lossless) for transmission with or without someloss of quality. The data reduction can be performed, for example, by acodec by leveraging information redundancy in the audio data, usingmethods such as coding, pattern recognition, and linear prediction toreduce the amount of information used to represent the uncompressedaudio data. The second type of compression is an audio levelcompression, in which the difference between a loud portion and a quietportion of an audio waveform is reduced, for example, by compressing orreducing the number of bits used for representing the audio waveform.The range of audio levels that may be represented using a given numberof bits used in a system can be referred to as the dynamic range of thesystem. In some implementations, a single codec may perform both typesof compressions described above.

A schematic example of system 100 including two wireless acousticearbuds is shown in FIG. 1. The system 100 includes a set of twoacoustic earbuds (105 a, 105 b, 105 in general) that are connected toone another over a wireless link 110. At least one of the acousticearbuds is connected to a source device 115 that generates audio signalsto be output by the earbuds 105. The connection between an earbud 105and the source device 115 can be over a wireless channel 120 such as aBluetooth® or Wi-Fi link. Because the wireless channel 120 isestablished over the environment, a latency associated with the channel120 can depend on various factors such as a physical length of thechannel or one or more environmental parameters that affect datatransmission over the channel. Accordingly, transmitting audio signalsseparately to the two earbuds 105 may result in a latency mismatch. Insome implementations, the latency mismatch can be addressed bytransmitting audio signals to one of the earbuds (105 a in the exampleof FIG. 1), which then transmits at least a portion of the audio signalsto the second earbud (105 b in the example of FIG. 1). This situation isdepicted in the example of FIG. 1, where the source device 115 transmitsaudio signals over the wireless channel 120 to the earbud 105 a, whichthen transmits a portion of the received signal to the earbud 105 b overthe wireless link 110.

The example of FIG. 1 shows the two earbuds 105 a and 105 b as discretein-ear devices. However, the terms earbud or acoustic earbud, as used inthis document, include various types of other personal acoustic devicessuch as in-ear, around-ear or over-the-ear headsets, earphones, andhearing aids. The earbuds 105 may be physically tethered to each other,for example, by a cord, an over-the-head bridge or headband, or abehind-the-head retaining structure.

The earbud that receives the signal from the source device 115 can bereferred to as a master, while the other earbud can be referred to as aslave. In some implementations, one of the earbuds can always functionas the master while the other earbud always functions as the slave. Inother implementations, the master earbud can be selected based on one ormore criteria such as signal strength. For example, if a user places thesource device (e.g., a smartphone) 115 in his left pocket, the leftearbud may receive a stronger signal than the right earbud, andtherefore be selected as the master. If the user puts the source device115 in the right pocket, or another location where the right earbudreceives a stronger signal, the roles of the master and slave may bereversed.

The source device 115 can include any device capable of generating audiosignals and transmitting them to the earbuds 105. For example, thesource device 115 can be a mobile device such as a smartphone, a tabletcomputer, an e-reader, or a portable media player. The source device 115can also be a portable or non-portable media playing device such as aTV, a disk-player, a gaming device, a receiver, a media streamingdevice, or a set-top box. In some implementations, the source device 115can be an intermediate device (e.g., a remote controller) thatinterfaces between a media player and the earbuds 105. In someimplementations, the source device 115 can be a hearing assistancedevice.

In some implementations, the source device 115 includes a transceiverthat can establish the wireless channel 120. In some implementations,the wireless channel 120 is established in accordance with a Bluetooth®Basic Rate/Enhanced Data Rate (BR/EDR) or Bluetooth® Low Energy (BLE)connections. For brevity, Bluetooth® Basic Rate/Enhanced Data Rate(BR/EDR) is interchangeably referred to herein as Bluetooth®. In someimplementations, the wireless channel 120 is established in accordancewith another communication protocol such as Near Field Communications(NFC), IEEE 802.11, or other local area network (LAN) or personal areanetwork (PAN) protocols. In some implementations, the wireless channel120 may support full bandwidth audio such as 48 KHz music.

The wireless link 110 is a link established between two acoustic devicessuch as the two earbuds 105. Examples of technologies that may be usedin establishing the wireless link 110 include Near-Field MagneticInduction (NFMI), Bluetooth®, BLE, Wi-Fi, or another personal areanetwork (PAN) protocol such as body area network (BAN), ZigBee, orINSTEON. In some implementations, such as for wireless earbuds, at leasta portion of the wireless link 110 is established through a human head.In some implementations, the data transfer capacity of the wireless link110 can be less than that of the wireless channel 120, thereby requiringadditional processing at the master device. The additional processingcan include audio compression that may be performed, for example, by anaudio codec. The additional processing can also include audio levelcompression, including, for example, adjusting a dynamic range of thetransmitted audio signals such that the signals can be transmitted usinga lower number of bits.

FIG. 2 is a block diagram of the system 100, the block diagramillustrating codecs 205 a and 205 b (205, in general) in the earbuds 105a and 105 b, respectively. The earbuds 105 a and 105 b also includeacoustic transducers 215 a and 215 b, respectively, for generating audiooutput, and one or more receivers 220 a and 220 b, respectively, forreceiving signals from the source device 115. In some implementations,where the wireless channel 120 is a Bluetooth® connection, the audiosignals can be transmitted from the source device 115 using, forexample, a Bluetooth® profile such as Advanced Audio DistributionProfile (A2DP). An A2DP profile can allow transfer of an audio stream inup to 2 channel stereo, from the source device 115 to the master earbud105 a. In some implementations, the A2DP stream can be encoded usingcodecs such as a sub-band coding (SBC) codec or Advanced Audio Coding(AAC) codec. In some implementations, other Bluetooth® profiles such asGeneric Audio/Video Distribution Profile (GAVDP) can also be used inplace of A2DP.

Audio signals received at the master earbud (105 a, in this example)from the source device 115 over the wireless channel 120 can beprocessed by the codec 205 a and output through a corresponding acoustictransducer 215 a. The codec 205 a can also process at least a portion ofthe audio signal and transmit the processed portion to the slave earbud(105 b, in this example) over the wireless link 110. At the slave side,the slave codec 205 b then processes the received signal and generatesaudio output through the corresponding acoustic transducer 215 b.

The codec 205 can be of various types. In some implementations, thecodec 205 can be an Adaptive Differential Pulse-Code Modulation (ADPCM)codec such as the ITU G.722 codec. The ITU G.722 is a wideband audiocodec that can be configured to operate at 48, 56 and 64 kbit/s. Otherexamples of codecs that may be used include SBC codec, FastStream, andAPTx. In some implementations, the codecs can be implemented on aprocessing device such as a digital signal processing (DSP) chip or aSystem-on-Chip (SoC). An example of a SoC includes CSR 8670 developed byCSR plc., a subsidiary of Qualcomm Inc. If a DSP or SoC is included inthe earbuds 105, additional processing of the audio stream received overthe wireless channel 120 can be performed before transmitting a portionof the received stream over the wireless link 110.

In some cases, using a codec 205 can pose challenges. For example, anADPCM codec such as the G.722 may add internally generated noise to thesignal processed by the codec. This may be manifested, for example, as abroadband residual noise at the output of the codec. This is oftenreferred to as codec noise or codec-generated noise, and may degrade thelistening experience, particularly at low volume levels. In addition,the bit rate compression carried out by the codec 205 may result in adynamic range reduction of the audio signal, which in turn may bemanifested as a perceived loss of audio quality. As an example, a G.722codec typically generates audio samples of 14 bits to reduce thetransmitted bit rate. This equates to about 12 dB of dynamic rangereduction as compared to the 16 bit samples of audio that may bereceived over the wireless channel 120. In some implementations, thetechnology described herein may be used to reduce the adverse effectsdue to the codec noise and dynamic range reduction issues associatedwith the use of a codec 205.

In some implementations, the codec noise internally generated by a codec205 can result in an audible noise being output by a correspondingacoustic transducer 215. In some cases, this may be particularlynoticeable when used in conjunction with an Asynchronous Sample RateConvertor (ASRC), which is used in wireless transmission systems tosynchronize the clocks between the transmitter and receiver devices suchas the master and slave earbuds, respectively. FIGS. 3A and 3B showplots illustrating degradation of signal due to codec noise.Specifically, FIG. 3A shows time domain waveforms for signals in thepresence and absence of a codec. The trace 305 shows a signal processedby a codec system that includes an ASRC, and the trace 310 shows thesame signal in the absence of the codec system. FIG. 4B shows thecorresponding spectral plots where the traces 315 and 320 representsignals in the presence and absence of the codec system, respectively.From FIGS. 3A and 3B, it can be seen that a significant amount ofbroadband codec noise is added by the codec system. Such codec noise canbe substantially independent of the input level and may be present evenin the absence of any input. This can result in a white noise beingoutput by the acoustic transducers 215 even when the source device 115is not transmitting any signal over the wireless channel 120. However,given that the noise is substantially independent of the input level,the noise may be masked at high input levels.

FIGS. 4A-4C are block diagrams of systems for reducing codec noise. FIG.4A shows a system 400 in which the input signal S at the master earbud105 a is processed by a pre-amplifier 405 that introduces apre-amplification gain of G such that the input to the codec 205 can berepresented as G.S. Assuming the transfer function of the codec 205 tobe unity, the output signal transmitted is approximately equal to G.S+N,wherein N is the codec noise added by the codec 205. For the sake ofbrevity, this example ignores the differences in quality between theinput and output of the codec 205. In order to compensate for thepre-amplification introduced by the pre-amplifier 405, the slave earbud105 b includes a post-amplifier 410 that introduces a post-amplificationgain of 1/G to the signal transmitted to the slave earbud 105 b.Therefore, the output at the slave earbud 105 b is given by:

$\begin{matrix}\begin{matrix}{{Output} = {\left( {1\text{/}G} \right)\left( {{G.S} + N} \right)}} \\{= {S + {N\text{/}G}}}\end{matrix} & (1)\end{matrix}$

Therefore, the noise component N is attenuated by the factor G, and theoverall improvement in signal to noise ratio is G.(S/N), which may beexpressed in dB as 20.log (G).

The output noise level can therefore be reduced to a target level byappropriately selecting the value of G. Even though a high value of Gmay be preferred, in practice, an upper limit of G may depend onavailable head room in the dynamic range of the digital signalprocessor. For example, a large value of G may in some cases clip theinput to the codec, thereby causing distortion. The value of G cantherefore be chosen with respect to a trade-off between input distortionand output codec noise. For example, the value of G can be chosen to behigh enough to prevent the codec noise from significantly distorting theperceived output audio quality, but also not so high as to cause theinput to the codec to be clipped.

In some implementations, the available headroom in the dynamic range canbe an unknown quantity, for example, because it depends on the level ofthe input audio content. In some cases, a low value of G may still causethe input to the codec to be clipped. Therefore, additional strategiesmay be used to reduce the chances of the audio being distorted due to aselected value of G. In some implementations, the maximum volume levelthat can be handled by the system (e.g., the earbud 105 a) can be set ata lower level than the capability of the corresponding system. Thislevel may be set, for example, in accordance with a selected value of Gsuch that the input S to the system is at a level that would not beclipped due to the selected value of G. In some cases however, suchlowering of overall volume level may not be acceptable, for example, ifthe audio output by the acoustic transducers are too low for a user. Insuch cases, a compressor may be used to process the output of thepre-amplifier before such output is provided to a codec.

FIG. 4B shows an example system 450 in which a compressor 410 can bepositioned between the pre-amplifier 405 and the codec 205 to reduce thepossibility of the input to the codec 205 being clipped. The compressor410 can perform audio level compression on the pre-amplified signal suchthat the adjusted pre-amplified signal substantially fits the dynamicrange of the codec without being clipped (or without being clipped to anextent that creates noticeable distortions). In some implementations,the compressor 410 may be used in conjunction with a reduction in themaximum volume level that can be handled by the system. In such cases,the use of a compressor 410 may allow for a lower reduction in themaximum volume level thereby allowing for higher values of G to bechosen for the pre-amplifier. This can be used, for example, to make useof the dynamic range headroom available for typical audio content whileincorporating a safety mechanism in the audio path for occasionalatypical content that might cause clipping and distortion.

In the example of FIG. 4B, using a compressor 410 can allow for themaximum volume of the system 450 to be pushed higher than what ispossible in the example of FIG. 4A. This may be possible, for example,by compensating for the pre-amplification G by a compression processperformed by the compressor 410. However, pushing the maximum volume toohigh for a high value of G can in some cases result in over-compressionof the audio signal, which in turn may lead to a perceptible loss insignal quality. In some implementations, this may be mitigated, at leastto some extent, using an adaptive system, an example 460 of which isshown in FIG. 4C. The system 460 includes a pre-amplifier 470, anamplification factor of which can be adjusted in accordance with afeedback signal 475 from the compressor 410. Such a system may be usedfor an efficient use of the available headroom to generate high qualityaudio playback while potentially reducing the codec noise toimperceptible levels.

In some implementations, the pre-amplification gain G is adaptivelyvaried in accordance with an operating state of the compressor 410. Forexample, the compressor 410 may track the incoming signal level and theavailable dynamic range headroom before applying compression, and changean operating state accordingly. Information about these operating stateparameters can be fed back to the pre-amplifier 470 via the feedbacksignal 475, and in turn used by the pre-amplifier 470 to adjust thepre-amplification gain G. For instance, if the compressor is close to apre-determined over compression threshold, the pre-amplification G canbe reduced (with an associated time constant) until a target compressoroperating state is achieved. Conversely if the input audio level is toolow, the pre-amplification gain G can be ramped up (with an associatedtime constant) to achieve a target operating state for the compressor410. In some implementations, the feedback signal 475 can include one ormore additional operating state metrics (e.g., a metric indicating thatthe system is getting close to the compressor limit) that may be used inadjusting the pre-amplification gain. In some cases, such additionalmetrics may be used to achieve a target overall system dynamics.

The adaptively selected pre-amplification gain G can be communicated tothe receiving device (e.g., the slave earbud 105 b) such that thepost-amplification gain of the corresponding post-amplifier 410 can beadjusted accordingly. This can be done, for example, using a feedforwardsignal 480 that may be transmitted over the wireless channel between thetwo acoustic devices. In some implementations, by setting thepost-amplification gain to be an inverse of the pre-amplification gainG, the overall system path gain may be held substantially close tounity. In some implementations, this can include adjusting the gain ofthe post-amplifier 410 after a time period to account for anypropagation delay between the pre-amplifier 470 and the post-amplifier410. In some implementations, an independent audio level detector may beused at the receiving acoustic device for determination of thepost-amplification gain. In some cases, a feedforward signal 480 maythus be avoided.

In implementations where the acoustic devices (e.g., the earbuds 105)process and output stereo audio, a dynamic gain adjustment on one of thestereo channels may be duplicated synchronously on the other channel,for example, to maintain a target audio spatial image. For such stereoaudio, the corresponding codec systems may also introduce, for example,a time delay and a non-flat frequency response that may be equalizedbetween the channels to maintain the target audio spatial image.

FIGS. 5A and 5B show plots illustrating reduction of codec noise usingtechnology described herein. Specifically FIG. 5A shows the spectrum 505of codec noise in the absence of the technology described herein, ascompared to a spectrum 510 of a direct wired equivalent signal in theabsence of a codec. The spectra were generated for a 2 KHz tone. FIG. 5Bshows the spectrum 515 of the codec noise compared to the same baselinespectrum 510 when the codec noise was reduced using the system 400depicted in FIG. 4A. In this example, a pre-amplification gain of G=3was used to improve the SNR by approximately 10 dB.

Using a pre-amplification gain to boost an input signal to a codec, andcompensating for the gain at a receiving device may therefore reduce atleast some adverse effects of associated codec noise. Another source ofcodec-introduced distortion can be a reduction in dynamic range due to alower number of bits used in a codec. For example, if an acoustic device(e.g., a master earbud 105) receives audio signals from a source deviceover a Bluetooth® link, the corresponding data samples may berepresented using 16 bits. On the other hand, a codec such as the ITUG.722 codec uses 14 bits to represent the data samples. Compared to the16 bit audio, this results in about 12 dB of reduction in the availabledynamic range. While such a reduction may be acceptable for some audiosignals (e.g., speech), the reduction may cause a perceptible loss inaudio quality in some high dynamic range content such as music. In someimplementations, distortions due to such reduction in available dynamicrange may be mitigated, at least in part, by selecting thepre-amplification gain to efficiently use the available dynamic range.In some cases, this may help in producing a high quality audioreproduction by an acoustic device that uses a codec.

FIG. 6A shows schematic illustrations of dynamic range reduction due tocertain codec usage. Plot 605 illustrates a dynamic range map of a 16bit input audio signal. The 16 bit audio has a dynamic range ofapproximately 96 dB, of which 84 dB is assumed to be occupied by theinput audio signal 610 in this example. Accordingly, a headroom 615 of12 dB is available on the input side of the codec 205. Due to the 12 dBreduction associated with the 14 bit codec 205, 12 dB of availabledynamic range is lost, as illustrated by the plot 620 on the output sideof the codec 205. This causes the entire signal to be shifted down,thereby causing a 12 dB segment 625 at the bottom to be lost. This inturn results in only 72 dB of effective signal dynamic range 630 at theoutput. The headroom 615 however still remains unaffected at 12 dB. Theexample of FIG. 6A thereby represents an inefficient usage of thedynamic range of the system. In some implementations, thepre-amplification gain G applied to the input signal to the codec 205can be selected to promote a more efficient use of the available dynamicrange.

FIG. 6B shows schematic illustrations of improvement in dynamic rangereduction. In the example, the input signal from the example of FIG. 6Ais boosted by a pre-amplification gain value G. In this example G=3,which corresponds to 10 dB of pre-amplification 645. The resultingdynamic range map 635 shows that the available headroom 640 is reducedby an equivalent amount (10 dB), resulting in an available headroom of 2dB at the input of the codec 205. The 14 bit codec 205 (ITU G.722, inthis example) shifts down the signal by 12 dB, and the plot 650illustrates the resulting signal dynamic range at the output of thecodec 205. The final dynamic range map 660 at the output of apost-amplifier 410 shows the signal at the original level with acorresponding loss 655 in the effective dynamic range. In this example,the post-amplifier 410 introduces a post-amplification gain factor of1/G and other equalization parameters as needed to maintain unity gainthrough the entire signal chain. Because the 12 dB discarded at theoutput of the codec 205 includes the 10 dB pre-amplification 645, theportion of the input audio signal 610 that is discarded in the final map660 amounts to only 2 dB. This represents a 10 dB improvement in theamount of potentially useful signal discarded, as compared to thecorresponding 12 dB in the example of FIG. 6A.

Therefore, the pre-amplification gain (and consequently in some cases,the post-amplification gain, and one or more equalization parameters)may be selected based on the available headroom to improve the dynamicrange performance of corresponding acoustic devices. In some cases, theheadroom management, as described with reference to FIG. 6B, may beimplemented in conjunction with the codec-noise reduction techniques(e.g., the pre- and post-amplification gain adjustment), for example, asa trade-off between headroom and perceived audio quality. Even thoughselecting a pre-amplification gain for one purpose (e.g., to reducecodec noise) may naturally help the other purpose (efficient use ofdynamic range, in this example), the operating parameters of anassociated system (e.g., the pre-amplification gain) may be selectedbased on the purpose that is of higher importance in a givenapplication.

FIG. 7 is a flowchart of an example process 800 for reducing codec noisethat may be generated by, for example, the codec 205 of FIGS. 4A-4C. Insome implementations, at least a portion of the process 700 may beexecuted by one or modules of an acoustic device. For example, theprocess 700 may be executed, at least in part, by one or more modules ofthe master earbud 105 a and/or slave earbud 105 b, as illustrated inFIGS. 4A-4C. Operations of the process 700 include receiving, at a firstacoustic device, a representation of an audio signal (710). The firstacoustic device can be an earbud such as the master earbud 105 adescribed above. The audio signal can include signals for two or moreaudio channels. For example, the audio signal can include an A2DP datastream that includes data corresponding to audio signals for twochannels (e.g., left speaker and right speaker) of an audio system. Theaudio signal can be generated by a source device such as the sourcedevice 115 described with reference to FIG. 1. The source device caninclude a media playing device capable of generating the audio signal inaccordance with a Bluetooth® profile such as the A2DP profile.

Operations of the process 700 also include amplifying the representationof the audio signal by a first gain factor to generate an amplifiedinput signal (720). The first gain factor can be substantially equal tothe pre-amplification gain described above with reference to FIGS.4A-4C. In some implementations, the first gain factor can be selectedsuch that the amplified input signal does not exceed a dynamic range ofa system of which the first acoustic device is a part. For example, thefirst gain factor can be selected such that the selected gain factor iscompatible with corresponding dynamic ranges of various devices andchannels in the overall audio path of the system.

In some implementations, the first gain factor can be selectedadaptively in accordance with the received representation of the audiosignal. For example, this can include detecting a dynamic range of thereceived representation of the audio signal, and determining availabledynamic range headroom. The headroom can be determined as a differencebetween a dynamic range of the system, and the dynamic range of thereceived representation of the audio signal. The first gain factor canthen be selected such that an associated amount of amplification doesnot exceed the available dynamic range headroom.

Operations of the process 700 further include processing the amplifiedinput signal by an audio codec to generate a processed signal thatrepresents a portion of the audio signal to be output by a secondacoustic device (730). The processed signal includes codec-noiseoriginating at the audio codec. In some implementations, the audio codeccan be an adaptive differential pulse code modulation (ADPCM) codec. Theprocessed signal can include noise originating at an asynchronous samplerate converter that is used in conjunction with the audio codec. In someimplementations, prior to processing by the audio codec, the amplifiedinput signal is compressed in accordance with a dynamic range of thesystem.

Operations of the process 700 also include transmitting the processedsignal to the second acoustic device (740). The second acoustic devicemay then receive a representation of the processed signal, andcompensate for the first gain factor by amplifying the representation ofthe processed signal by a second gain factor to generate a compensatedsignal. The compensated signal is then output at the second acousticdevice through an acoustic transducer. The second acoustic device can bean acoustic earbud such as the slave earbud 105 b described above. Thesecond gain factor can be substantially equal to the post-amplificationgain described above. For example, the second gain factor can besubstantially an inverse of the first gain factor. In someimplementations, information indicative of an adaptively selected gainfactor may be transmitted to the second acoustic device through awireless link.

The functionality described herein, or portions thereof, and its variousmodifications (hereinafter “the functions”) can be implemented, at leastin part, via a computer program product, e.g., a computer programtangibly embodied in an information carrier, such as one or morenon-transitory machine-readable media or storage device, for executionby, or to control the operation of, one or more data processingapparatus, e.g., a programmable processor, a computer, multiplecomputers, and/or programmable logic components.

A computer program can be written in any form of programming language,including compiled or interpreted languages, and it can be deployed inany form, including as a stand-alone program or as a module, component,subroutine, or other unit suitable for use in a computing environment. Acomputer program can be deployed to be executed on one computer or onmultiple computers at one site or distributed across multiple sites andinterconnected by a network.

Actions associated with implementing all or part of the functions can beperformed by one or more programmable processors executing one or morecomputer programs to perform the functions of the calibration process.All or part of the functions can be implemented as, special purposelogic circuitry, e.g., an FPGA and/or an ASIC (application-specificintegrated circuit).

Processors suitable for the execution of a computer program include, byway of example, both general and special purpose microprocessors, andany one or more processors of any kind of digital computer. Generally, aprocessor will receive instructions and data from a read-only memory ora random access memory or both. Components of a computer include aprocessor for executing instructions and one or more memory devices forstoring instructions and data.

Other embodiments and applications not specifically described herein arealso within the scope of the following claims. For example, in caseswhere the codec generated noise has a non-uniform spectral distributionfrequency shaping may also be applied to the pre-amplifier gain. In somecases, such frequency shaping may help in avoiding boosting the signalunnecessarily in frequency regions where the noise is low orimperceptible. This can be used, for example, in situations where thecodec generated noise has significant high frequency energy, whereastypical audio content has a relatively small crest factor at suchfrequencies, and hence a larger headroom to spare.

Elements of different implementations described herein may be combinedto form other embodiments not specifically set forth above. Elements maybe left out of the structures described herein without adverselyaffecting their operation. Furthermore, various separate elements may becombined into one or more individual elements to perform the functionsdescribed herein.

1. A computer-implemented method comprising: receiving, at a firstacoustic device, a representation of an audio signal; amplifying therepresentation of the audio signal by a first gain factor to generate anamplified input signal; processing the amplified input signal by anaudio codec comprising one or more processors to generate a processedsignal that represents a portion of the audio signal to be output by asecond acoustic device remote with respect to the first acoustic device,wherein the processed signal includes noise originating at the audiocodec; and transmitting the processed signal to the second acousticdevice over a wireless link, wherein the processing comprises bit ratecompression in accordance with a data transfer capacity of the wirelesslink.
 2. The method of claim 1, further comprising: receiving, at thesecond acoustic device, a representation of the processed signal;compensating for the first gain factor by amplifying the representationof the processed signal by a second gain factor to generate acompensated signal; and outputting the compensated signal at the secondacoustic device.
 3. The method of claim 2, wherein the second gainfactor is substantially an inverse of the first gain factor.
 4. Themethod of claim 1, wherein the audio codec is an adaptive differentialpulse code modulation (ADPCM) codec.
 5. The method of claim 1, whereinthe processed signal also includes noise originating at an asynchronoussample rate converter used in conjunction with the audio codec.
 6. Themethod of claim 1, wherein the first gain factor is selected such thatthe amplified input signal does not exceed a dynamic range of a systemin which the audio codec is disposed.
 7. The method of claim 1, furthercomprising compressing the amplified input signal in accordance with adynamic range of a system in which the audio codec is disposed.
 8. Themethod of claim 1, wherein the first gain factor is selected adaptivelyin accordance with the received representation of the audio signal. 9.The method of claim 8, comprising: detecting a dynamic range of thereceived representation of the audio signal; determining availabledynamic range headroom as a difference between a dynamic range of asystem in which the audio codec is disposed, and the dynamic range ofthe received representation of the audio signal; and selecting the firstgain factor such that an associated amount of amplification does notexceed the available dynamic range headroom.
 10. The method of claim 8,further comprising transmitting information indicative of the adaptivelyselected gain factor to the second acoustic device.
 11. The method ofclaim 1, wherein the first and second acoustic devices are earphonesthat are connected to one another over a wireless link.
 12. The methodof claim 1, wherein the first gain factor is a function of frequency.13. A system comprising: a first acoustic device configured to receive arepresentation of an audio signal, the first acoustic device comprising:an amplifier configured to amplify the representation of the audiosignal by a first gain factor to generate an amplified input signal, anaudio codec comprising one or more processors, the audio codecconfigured to process the amplified input signal to generate a processedsignal that represents a portion of the audio signal to be output by asecond acoustic device remote with respect to the first acoustic device,wherein the processed signal includes noise originating at the audiocodec, and a transmitter for transmitting the processed signal to thesecond acoustic device over a wireless link, wherein the processingcomprises bit rate compression in accordance with a data transfercapacity of the wireless link.
 14. The system of claim 13, furthercomprising: the second acoustic device configured to receive arepresentation of the processed signal, the second acoustic devicecomprising: an amplifier configured to compensate for the first gainfactor by amplifying the representation of the processed signal by asecond gain factor to generate a compensated signal, and a receivercodec configured to process the compensated signal to output thecompensated signal at the second acoustic device.
 15. The system ofclaim 14, wherein the second gain factor is substantially an inverse ofthe first gain factor.
 16. The system of claim 13, wherein the audiocodec is an adaptive differential pulse code modulation (ADPCM) codec.17. The system of claim 13, wherein the processed signal also includesnoise originating at an asynchronous sample rate converter used inconjunction with the audio codec.
 18. The system of claim 13, whereinthe first gain factor is selected such that the amplified input signaldoes not exceed a dynamic range of the system.
 19. The system of claim13, further comprising a compressor for compressing the amplified inputsignal in accordance with a dynamic range of the system.
 20. The systemof claim 13, wherein the first gain factor is selected adaptively inaccordance with the received representation of the audio signal.
 21. Thesystem of claim 20, wherein the first acoustic device is furtherconfigured to: detect a dynamic range of the received representation ofthe audio signal; determine available dynamic range headroom as adifference between a dynamic range of the system and the dynamic rangeof the received representation of the audio signal; and select the firstgain factor such that an associated amount of amplification does notexceed the available dynamic range headroom.
 22. The system of claim 21,wherein information indicative of the adaptively selected gain factor istransmitted to the second acoustic device.
 23. The system of claim 13,wherein the first and second acoustic devices are earphones that areconnected to one another over a wireless link.
 24. The system of claim13, wherein the first gain factor is a function of frequency.
 25. Amachine-readable storage device having encoded thereon computer readableinstructions for causing one or more processors to perform operationscomprising: receiving a representation of an audio signal; amplifyingthe representation of the audio signal by a first gain factor togenerate an amplified input signal; processing the amplified inputsignal to generate a processed signal that represents a portion of theaudio signal to be output by an acoustic device that is remote from theone or more processors, wherein the processed signal includes noiseoriginating at an audio codec; and providing the processed signal fortransmission to the acoustic device over a wireless link, wherein theprocessing comprises bit rate compression in accordance with a datatransfer capacity of the wireless link.